Of motors for vehicles, a motor to be used for driving a car is demanded for reduction in size and increase in output power. Accordingly, the use of a flat rectangular conductor effective in improvement of a space factor has been discussed.
However, when the rectangular conductor is to be used for a coil, it is hard to wind the rectangular conductor in a coiled form due to its wide cross-sectional area. Although a wider cross-sectional area of the rectangular conductor can lead to an increase in current density, a problem with an eddy current arises. Therefore, in forming a coil by winding the rectangular conductor, various reviews or studies have been made.
Patent Document 1 discloses a technique related to a rectangular wire structure, a winding method for rectangular wire, and a winding device.
In the case of winding the rectangular conductor in a multilayered form, using a winding device, a rectangular wire may be laterally dislocated or displaced during winding. In Patent Document 1, to avoid such a problem, the rectangular conductor is partially formed with a recessed or protruding retaining portion and is wound. With this configuration, it is possible to prevent displacement of the rectangular conductor to be wound as second and subsequent layers on the rectangular conductor.
Patent Document 2 discloses a technique related to a winding structure of an electric motor, a winding method and a winding apparatus.
Positioning means is provided to position a rectangular conductor so as to incline a cross section of the conductor at a predetermined angle relative to a line passing the center of a teeth portion of a stator core. The rectangular conductor is wound in a multilayered configuration to form a coil. This positioning means is constituted of an insulator with a stepped surface to retain the rectangular conductor at a slant along the stepped surface. With this configuration, it is possible to prevent positional displacement of the rectangular conductor.
Patent Document 3 discloses a technique related to a stator structure for a rotary electric machine and a manufacturing method of the stator structure.
A pair of rectangular conductors are wound on a teeth portion of a stator core to form a two-layered coil. At that time, assuming that the number of rectangular conductors to be supplied to each teeth portion is P, the number of slots of an entire stator is T, and the number of neutral points is S, the winding wire is twisted between teeth portions at intervals of N pieces that satisfy the relation: T=3×S×P×N.
With such configuration, a pair coil can be made from a rectangular conductor. This makes it possible to suppress loss of cyclic currents and others and reduce the cross-sectional area per one rectangular conductor, thus preventing the occurrence of eddy currents or the like.
However, the method of forming a coil by winding a rectangular conductor as disclosed in Patent Documents 1 and 2 may involve the following problems.
There is first adopted a method of winding a conductor sequentially from a base side of a teeth portion of a stator core basically along the teeth surface to the inside in a radiation direction of the core, and then returning back at a distal end side of the teeth portion to form a second layer. This is regarded as a most general method to wind a coil by use of a winding device.
However, in this winding manner, a returning point of a second layer is located on the base side of the teeth portion. Thus, a winding start portion of the first layer and a winding end portion of the second layer overlap one on the other. A potential difference between a winding start portion and a winding end portion becomes highest when a coil is supplied with currents. Accordingly, in a two-layered coil, a potential difference is highest between a winding start portion of a first layer and a winding end portion of a second layer.
Therefore, a rectangular conductor has to be applied with insulating coating enough to withstand this potential difference and thus it is conceivable that the rectangular conductor needs to be coated with thick insulating coating.
As the thickness of the insulating coating is thicker, however, the space factor decreases, which may inhibit the increase in output power of a motor.
On the other hand, in the stator of Patent Document 3 with the rectangular conductor being wound in pairs, such a problem as in Patent Documents 1 and 2 will not occur. However, a mechanism of a winding device for pair winding is apt to be complex and also the peripheral length of the pair coil on an outer circumferential side is longer than on an inner circumferential side. This causes a resistance difference, which may greatly generate heat in the coil.
Consequently, it may inhibit the increase in output power of a motor.
As a method to solve the problem with potential difference in Patent Documents 1 to 3, the use of a winding method as disclosed in Patent Document 4 is conceivable.
FIG. 10 shows a cross section of a coil in Patent Document 4.
A coil 200 is a wound coil in two layers and four rows (2-layer×4-row) as shown in FIG. 10 by winding a wire to sequentially form an outer layer, an inner layer, another inner layer, and another outer layer. As a result of such winding, a potential difference of adjacent conductors merely occurs by an amount corresponding to four turns. Thus, a reduction in potential difference can be achieved.
As the potential difference is lower, the thickness of the insulating coating to be provided around the conductor can be made thinner. This can realize compact size and high power output of the coil 200.